If I take your example as what you are wishing to achieve then : Unless you are a kde guru : Forget it !

As a stupid example : kdelibs is almost probably the package that takes the longest time to build, but it is a dependency of many kde packages.
What is the point of updating packages depending on kdelibs and not updating the kdelibs, being said that in the worst case, it can just break ?

On a more general standpoint : Of course you can do that after having cared of all the dependencies.
But the recommended way to do this would be using portage's ressources (package.mask / package.provided / your own ebuilds in you local overlay) rather than fiddling regexes._________________